In recent years, along with the improvement in performance of automobiles and machine structures, much higher fatigue properties, more complicated part shapes, and more improved deformability of the steel material have been strongly demanded. The steel pipe which is used as material for auto parts, machine structure parts, etc. is cold worked, then machined to obtain the final part shape and adjusted in strength by subsequent quenching. Hardened auto parts and machine structure parts are required to have strength and fatigue properties.
To raise the strength of steel pipe by quenching, it is necessary to raise the C content of the steel pipe material. However, high carbon steel is inferior in deformability, so the method of lowering the strength by spheroidization heat treatment has been proposed (for example, see PLT's 1 to 4).
On the other hand, to improve the fatigue properties, in general it is effective to increase the hardness or strength or to impart residual stress of compression. Up until now, the method of using carburization to increase the hardness and strength of the surface layer part so as to improve the fatigue properties (see PLT 5) has been proposed.
However, in recent years, from the viewpoint of lower costs and production efficiency, electric resistance welded steel pipe excellent in deformability which can be worked into complicated shapes even after omitting spheroidization heat treatment has been demanded. Further, it has been demanded to improve the fatigue properties of parts after quenching without carburization etc.
Furthermore, in electric resistance welded steel pipe used for applications in which deformability is demanded, in particular, Ca is added for controlling the morphology of MnS. Further, to make the quality of the base material and the weld zone uniform, the practice has been to use electric resistance welding to make the pipe, then perform normalizing (heating to the Ac3 transformation point or more and air cooling) or other heat treatment.
However, when omitting the heat treatment after pipemaking from the viewpoint of lower costs and production efficiency, according to studies of the inventors, there is the problem of deterioration of the deformability. In steel to which Ca is added, it is demanded that the deformability not be impaired even if omitting the heat treatment after pipemaking.